Structural health monitoring (SHM) is a collective term for technologies that are used to monitor the structural integrity of structures or identify structural damage by permanently attached sensors. In the past SHM techniques have been investigated for example for the aeronautical industry. This sector is eager to integrate SHM systems in their products due to the fact of a continuous search for enhancing the safety level of their products and reduction of the direct operating costs for operating an aircraft. Currently visual inspections and a large number of non-destructive evaluation techniques with a local inspection capability are available for the industry. In the last 3 decades numerous SHM techniques have been investigated with the focus on damage identification in components on a more global character. The basic scientific challenge for SHM remains to detect damage, which is a very local phenomenon by measuring global responses parameters of a structure. Next to this primary scientific challenge some other big barriers block the industry from applying SHM systems more in particular their effectiveness in practice and durability under in-service conditions.
These challenges are some of the major blocking points which have prevented the introduction of SHM technologies in real life structures/applications with exception of condition monitoring of rotary machinery. The maturity of different evolving SHM systems has been expressed in technological readiness levels by the large industrial players (e.g. Airbus) and governmental research organisation (e.g. NASA). The definitions vary slightly for the different organisations but the major philosophy remains the same. An example is illustrated in FIG. 1, as described by Wilwhite A. W. et al. in Workshop Proceedings, NASA Jet Propulsion Laboratory (JPL-Pub1-2004-011) (June) (2004), pp. 14-30, a detailed description of the technological readiness levels indicated being described in detail in this reference.
Fatigue is an important damage phenomenon that affects the structural integrity of a structural component/applications. Fatigue has been approached in many ways:
In the first half of 20th century, cyclic slip was considered to be essential for micro-crack initiation and it was also associated with cyclic dislocation movements. Micro-cracks usually start at the free surface of the material, also in unnotched objects under study with a nominally homogeneous stress distribution tested under cyclic tension. The restrain on cyclic slip is lower than inside the material because of the free surface at one side of the surface material. It can be concluded that fatigue crack initiation is a surface phenomenon. Fatigue life under cyclic loading consisted of two phases: crack initiation life and crack growth period until failure. The initiation period typically may start with cyclic slip, resulting in crack nucleation, which further results in micro crack growth. The first phase may cover a large part of the fatigue life under stress amplitudes just above the fatigue limit but for higher stress amplitudes the crack growth period is essential for the fatigue life. The crack growth period, which is the second phase, typically consists of macro crack growth ending in final failure. The difference between the two phases is of great importance because several surface conditions do affect the initiation period but not the crack growth period (surface roughness, surface damage, surface residual stresses, surface treatments). Corrosive environments and tribological phenomena can also affect initiation and crack growth period but in different way for the two periods. The stress concentration factor Kt is the important parameter for predictions on crack initiation, the stress intensity factor K is used for predictions on crack growth, while the fracture toughness Klc is characteristic for the final failure.
Fatigue properties can also be described in terms of a Wöler Curve (S-N curve). A S-N curve is derived from a number of fatigue tests at different stress levels, the fatigue life N is plotted on a logarithmic scale in the horizontal direction and the stress amplitude on a linear scale in the vertical direction. As can be seen in FIG. 2, available from J. Schijve, “Fatigue of structures and materials in the 20th century and the state of the art,” International Journal of Fatigue, vol. 25, no. 8, pp. 679-702, August 2003, for low stress amplitudes the curve exhibited a lower limit which implies that the failure has not occurred even after a high number of load cycles. The horizontal asymptote of the S-N curve is called the fatigue limit. At the upper side of the S-N curve, where the large stress amplitudes are, another horizontal asymptote appears. If failure did not occur in the first cycle, the fatigue life could be several hundreds of cycles. The ‘low-cycle fatigue’ area implies that macro plastic deformation occurs in every cycle. On the other hand, at lower stress amplitudes macro plastic deformation does not occur and the fatigue phenomenon is called as ‘high-cycle fatigue’.
Besides fatigue, the structural integrity of objects can also be impacted by wear (or tribological) phenomena. These phenomena are sometimes interrelated. Wear phenomena can play an important role for structural health of objects and therefore need to be monitored.
There is still room for systems and methods for structural health monitoring of objects, whereby the structural health monitoring system is reliable and robust and is not penalizing the useful physical integrity of the object under study.